MDEV-37187 hashicorp plugin: avoid expensive clock() in hot path

[vaintroub]

The plugin used clock() only to timestamp cache entries and measure elapsed time against millisecond timeouts. As shown in MDEV-12345, clock() is prohibitively expensive, and we do not need per-thread or per-process CPU time here, only time differences.

Replace clock()/clock_t with the monotonic std::chrono::steady_clock. Timeouts are kept as std::chrono::milliseconds

[gemini-code-assist]

Code Review

This pull request refactors the HashiCorp Key Management plugin to use std::chrono::steady_clock instead of clock_t and clock() for tracking cache timeouts and timestamps, ensuring monotonic and accurate real-time measurements. I have no feedback to provide as there are no review comments.

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